A basis to extend the proof of migration routes of immature parasites inside hosts: estimated time of arrival of Nippostrongylus brasiliensis and Strongyloides ratti in the gut of the rat.

The time taken for larvae of Nippostrongylus brasiliensis and a homogonic strain of Strongyloides ratti to complete their migration from the skin surface to the small intestine of the rat was estimated by a new method in which a mirror image of arrival times was created by counting mature parasites at day 8 (N. brasiliensis) or day 5 (S. ratti) in the intestines of rats that had received a single pulse of morantel tartrate by stomach tube at different times starting 10 h after skin application of exact doses of infective larvae. It was confirmed that the effect of the drug was confined to parasites in the gut, so that the kinetics of migration were unperturbed. The persistence of the anthelmintic effect in the gut was shown by independent assays to be less than 8 h for both species. The time-course of arrival calculated from drug pulse data corrected for persistent effects was compared with direct counts of larvae arriving in the intestines of rats not dosed with anthelmintic. Both methods agreed in all respects for N. brasiliensis, whereas the estimates for direct counts of S. ratti, lagged 15-17 h behind those from drug-treated rats. In the discussion it is argued that the anthelmintic method provides a more correct picture for S. ratti and, on this basis, rates and synchrony of migration of the two species are compared. The role of these data as part of a proof of migration routes is explained.